cli: sort does not duplicate channels and schemas on every message (#…

…1171)

### Changelog

- fixed: `mcap sort` would rewrite the schema and channel for every
message in the output file. This is fixed.

### Docs

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documentation changes are needed. -->

### Description

<!-- Describe the problem, what has changed, and motivation behind those
changes. Pretend you are advocating for this change and the reader is
skeptical. -->

<!-- In addition to unit tests, describe any manual testing you did to
validate this change. -->

<table><tr><th>Before</th><th>After</th></tr><tr><td>

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</td><td>

<!--after content goes here-->

</td></tr></table>

<!-- If necessary, link relevant Linear or Github issues. Use `Fixes:
foxglove/repo#1234` to auto-close the Github issue or Fixes: FG-### for
Linear isses. -->

go/cli/mcap/cmd/sort.go

@@ -123,7 +123,7 @@ func sortFile(w io.Writer, r io.ReadSeeker) error {
 		return fmt.Errorf("failed to read messages: %w", err)
 	}
 	schemas := make(map[uint16]*mcap.Schema)
-	channels := make(map[uint16]*mcap.Schema)
+	channels := make(map[uint16]*mcap.Channel)
 	message := mcap.Message{}
 	for {
 		schema, channel, _, err := it.NextInto(&message)
@@ -138,13 +138,15 @@ func sortFile(w io.Writer, r io.ReadSeeker) error {
 				if err != nil {
 					return fmt.Errorf("failed to write schema: %w", err)
 				}
+				schemas[schema.ID] = schema
 			}
 		}
 		if _, ok := channels[channel.ID]; !ok {
 			err := writer.WriteChannel(channel)
 			if err != nil {
 				return fmt.Errorf("failed to write channel: %w", err)
 			}
+			channels[channel.ID] = channel
 		}
 		err = writer.WriteMessage(&message)
 		if err != nil {

go/cli/mcap/cmd/sort_test.go

@@ -2,6 +2,8 @@ package cmd
 
 import (
 	"bytes"
+	"errors"
+	"io"
 	"testing"
 
 	"github.com/foxglove/mcap/go/mcap"
@@ -62,14 +64,32 @@ func TestSortFile(t *testing.T) {
 	w := &bytes.Buffer{}
 	require.NoError(t, sortFile(w, reader))
 
-	// verify it is now sorted
-	r, err := mcap.NewReader(bytes.NewReader(w.Bytes()))
+	lexer, err := mcap.NewLexer(bytes.NewReader(w.Bytes()))
 	require.NoError(t, err)
-
-	it, err := r.Messages(mcap.UsingIndex(false))
-	require.NoError(t, err)
-
-	_, _, msg, err := it.NextInto(nil)
-	require.NoError(t, err)
-	assert.Equal(t, 25, int(msg.LogTime))
+	var schemaCount, channelCount, messageCount int
+	var lastMessageTime uint64
+top:
+	for {
+		token, record, err := lexer.Next(nil)
+		if errors.Is(err, io.EOF) {
+			break
+		}
+		require.NoError(t, err)
+		switch token {
+		case mcap.TokenMessage:
+			messageCount++
+			message, err := mcap.ParseMessage(record)
+			require.NoError(t, err)
+			require.GreaterOrEqual(t, message.LogTime, lastMessageTime)
+			lastMessageTime = message.LogTime
+		case mcap.TokenSchema:
+			schemaCount++
+		case mcap.TokenChannel:
+			channelCount++
+		case mcap.TokenDataEnd:
+			break top
+		}
+	}
+	assert.Equal(t, 1, schemaCount, "incorrect schema count")
+	assert.Equal(t, 2, channelCount, "incorrect channel count")
 }